VCL OpenGL backend performance improvements

Transcript

1. www.CollaboraOffice.com tomaz.vajngerl@collabora.co.uk Collabora Productivity Tomaž Vajngerl @CollaboraOffice VCL OpenGL backend performance improvements
2. Work on VCL OpenGL backend started in 2014 (LibreOffice 4.4)
3. In LibreOffice 5.1 most annoying render bugs are fixed
4. First make it work, then make it fast Usual misconception: “It is hardware accelerated so it is fast.”
5. First make it work, then make it fast However rendering on GPU is different than how 2D rendering is done with typical “canvas” style API.
6. Rendering with GPU • Everything is composed of triangles • No immediate drawing (performance hit) • Upload objects (vertices, textures) to the GPU memory and reuse • Programmable rasterization (With fragment shaders)
7. Performance improvements
8. Native control cache • Native controls are rendered to a buffer, then uploaded as a texture • Expensive on each draw • Some controls never change, some change when resizing • We can cache them as textures
9. Texture atlas to increase texute drawing performance • Use one texture for more images • Packing • Use a simple algorithm – divide texture to equally sized regions • Highly dynamic but wastes space in texture • Useful for icons
10. Text rendering • No support to draw text on GPU so we must render text to texture and upload – slow • Instead render individual glyphs to texture atlas and reuse when drawing • Draw more glyphs of them with one draw call
11. Text rendering ABRACADABRA ABR C D
12. Decreasing state changes • Track bound textures – don’t unbind if not necessary • Track state of GL_SCISSOR_TEST, GL_STENCIL_TEST, GL_BLEND_TEST and don’t enable/disable if already enabled/disabled • Don’t change glViewport and glScissor if it already is set correctly
13. Combine shaders • Shader program switching is changing state • Combine shaders into bigger shaders • Non-texture drawing • Texture drawing • Shaders for scaling, gradient drawing, etc. • Switch between modes with a shader parameter (and switch or if inside shader)
14. Polyline drawing with GPU • Bezier curves • Open / closed • Line ending • Line joins
15. Polyline drawing with GPU • Trapezoid decomposition for a polyline on the CPU is expensive – we can draw lines on GPU • Anti-aliasing using shaders • Also used for line drawing, polypolygon and polygon outline and anti-aliasing
16. Polyline drawing with GPU Feather (0.5 pixel) ½ Line width Feather (0.5 pixel) ½ Line width Extrusion vector
17. Batching & combining • Decrease GPU overhead – reduce draw calls • Batch drawing to be able to reorder and combine same draw actions • Current state: • (Poly)Polygon, Rectangle, (Poly)line and text rendering is batched. • Gradient, most texture rendering is not (yet).
18. Batching & combining Draw Line (10, 20, 20, 30) Draw Rect (40, 10, 50, 20) Draw Rect (10, 10, 20, 20) Draw Line (30, 30, 30, 10) Draw Rect (0, 0, 60, 60)
19. Batching & combining Draw Line (10, 20, 20, 30) Draw Rect (40, 10, 50, 20) Draw Rect (10, 10, 20, 20) Draw Line (30, 30, 30, 10) Draw Rect (0, 0, 60, 60) Overlap
20. Batching & combining Draw Line (10, 20, 20, 30) Draw Rect (40, 10, 50, 20) Draw Rect (10, 10, 20, 20) Draw Line (30, 30, 30, 10) Draw Rect (0, 0, 60, 60) Overlap
21. Batching & combining Draw Line (10, 20, 20, 30) Draw Rect (40, 10, 50, 20) Draw Rect (10, 10, 20, 20) Draw Line (30, 30, 30, 10) Draw Rect (0, 0, 60, 60) Overlap Combine Combine
22. Batching & combining Draw Line (10, 20, 20, 30) Draw Line (30, 30, 30, 10) Draw Rect (10, 10, 20, 20) Draw Rect (40, 10, 50, 20) Draw Rect (0, 0, 60, 60) Overlap
23. Backend Testing
24. Visual backend test • Draw primitives to a virtual device • Check pixels if they match • Pass, Fail, Pass with quirks
25. Visual backend test • For finding rendering bugs in existing backends • Helpful to code new backends • First run test for OpenGL driver (when using OpenGL test)
26. Future improvements
27. (Filled) Polygon drawing with GPU • Draw with help of stencil buffer which covers • But this is mostly expensive • Not implemented – generally better to do it on the CPU
28. Bézier curves • Curve Rendering using GPU - Loop Blinn algorithm • Alternative: do decomposition with geometry shader
29. Make API more GPU frendly • Scenegraph API for VCL • Tree of objects, we can optimize for a rendering target • Matrix transform instead of modifying coordinates • Rendering thread
30. Thanks